doc: fix some grammar errors and refine some expressions

[caozhuozi]

In this PR, I've made the following changes:

1. grammar fixes:

   • missing subject

     - If the model has multiple weight files, need to package them in separate layers. But recommended [...]
     + If the model has multiple weight files, they SHOULD be packaged in separate layers.

2. removed the "The model specification needs to consider two factors" part.

   I don't think design considerations/motivations should be included in the specification document itself (but rather in a separate place such as README.md).

   I've transformed this information into a "Use Case" section, similar to the arrangement in the OCI Distribution Spec.

3. reordered the "Workflow" and "Spec Definition" sections:

   I think it's better to introduce the spec itself before discussing the workflow.

4. clarified the distinction between Model Format Spec and Manifest Spec:

   Several parts of the document blurred the distinction between these two concepts. For example,

   The model specification is defined through the artifact extension based on the OCI image specification, and extend model features through artifactType and annotations. Model storage and distribution can be optimized based on artifact extension.

   I attempted to explicitly differentiate them, as they represent concepts at different levels. For example,

   At a high level, the Model Format Specification is based on the OCI Image Format Specification and incorporates all its components. The key distinction lies in extending the OCI Image Manifest Specification to accommodate artifact usage specifically tailored for AI/ML models.

5. removed redundancies:

   Several parts of the document repeat the same information, For example,

   • regarding artifactType and annotations properties:

     [...], and extends model features through artifactType and annotations. (in # Model Format Specification)

     [...], and extend model features through artifactType and annotations. (in ## Overview)

     Now only mentioned once:

     Specifically, it leverages the extensible artifactType and annotations properties to define attributes specific to model artifacts.

   • regarding compression:

     [...], it's recommended to use the application/vnd.oci.image.layer.v1.tar format for the model layer to avoid compression (in ### BUILD & PUSH)

     [...] But recommended package model weight files without compressed to avoid the container runtime decompressing the model layer (in ### Image Manifest Extension Properties)

     Now consolidated as an implementers note, aligning with the writing conventions used in the OCI Image Specification.

     Implementers note: It is recommended to package weight files without compression to avoid unnecessary overhead of decompression by the container runtime as model weight files are typically already compressed.

6. added "REQUIRED" or "OPTIONAL" to all properties

   • Added "REQUIRED" to mediaType where it was missing.
   • Added "OPTIONAL" to annotations where it was missing.

[bergwolf]

LGTM, thanks! Please fix the lint error though.

[gaius-qi]

LGTM